In recent years, there has been a trend toward miniaturization of electronic products. However, under the precondition of increased power intensity, the component density of a miniaturized electronic product is increased, and the circuit layout thereof becomes denser, thereby incurring electromagnetic interference (EMI) among electronic components thereof. On the other hand, electromagnetic waves produced by the miniaturized electronic product may interfere with other electronic equipments and may even be harmful to human health. In order to solve such EMI problem, many countries regulate the levels of EMI emissions that are allowed to emanate from electronic products, and require testing of such electronic products to verify compliance with the regulations before releasing the products to the market.
FIG. 1 illustrates a conventional AC-to-DC power supply 9 that includes an input unit 91, a bridge rectifier 92 and a DC-to-DC converter 93. The input unit 91 includes a first input end (P1) and a second input end (P2) that are cooperatively used to receive an AC input voltage, and a grounded end (P3). The bridge rectifier 92 has two input terminals coupled respectively to the first and second input ends (P1, P2), and two output terminals (P4, P5) coupled to the DC-to-DC converter 93. The output terminal (P5) is grounded. The bridge rectifier 92 receives the AC input voltage from the first and second input ends (P1, P2) through the input terminals, converts the AC input voltage into a DC voltage, and outputs the DC voltage to the DC-to-DC converter 93 through the output terminals (P4, P5). The DC-to-DC converter 93 converts the DC voltage from the bridge rectifier 92 into a predetermined DC output voltage, and outputs the predetermined DC output voltage through two output ends (P6, P7) thereof. The output end (P7) is grounded. In this case, the output ends (P6, P7) cooperatively constitute an output end unit 94 for outputting the predetermined DC output voltage. Furthermore, a Y-type capacitor (Cy) is used to suppress electromagnetic interference. The Y-type capacitor (Cy) has one end coupled electrically to the output terminal (P5) of the bridge rectifier 92, and has another end coupled electrically to the output ends (P7) of the DC-to-DC converter 93 and the grounded end (P3) of the input unit 91.
In such a configuration, the larger the capacitance of the Y-type capacitor (Cy), the greater the effect of electromagnetic interference suppression. However, a high-frequency isolation transformer included in the DC-to-DC converter 93 may result in a leakage current that increases with increase of the capacitance of the Y-type capacitor (Cy). Therefore, even though electromagnetic interference is suppressed, such AC-to-DC power supply 9 may not meet the standards associated with the leakage current.